1. Technical Field
This invention generally relates to diffusers and, more particularly, to a diffuser used in a centrifugal compressor. Specifically, the present invention relates to a diffuser having a valve that selectively opens and closes the outlets of the diffuser to create an unsteady wave pattern that will simultaneously diffuse a centrifugal impeller outflow and turn it to an axial direction.
2. Background Information
A centrifugal compressor employs an impeller that rotates inside a stationary flowpath that is typically formed by an inlet, a shroud, a diffuser, and a discharge duct. The impeller draws a stream of gas through the inlet in a direction that is generally parallel to the axis of rotation of the impeller. A plurality of impeller blades then act upon the stream to impart kinetic energy to the stream. The stream exits the impeller in a direction that is orthogonal or substantially orthogonal to the impeller's axis of rotation. The diffuser then acts on the stream to convert the kinetic energy of the stream to static pressure before discharging the flow to a duct. Typically, the flow discharged from the diffuser must be redirected to a direction substantially parallel to the axis of rotation of the impeller before being delivered to subsequent components.
The flow discharging from centrifugal impeller has significant velocity components in the tangential and radial directions. The tangential velocity component is primarily a result of work input by the impeller. The radial component is a function of the mass moving through the impeller and the flow path area and static conditions at the impeller trailing edge.
Conventional diffusers operate in a steady flow environment and employ conservation of mass and momentum principles to recover kinetic energy from the impeller discharge flow by reducing the absolute Mach number of the flow. As Mach number is reduced, the static pressure and density of the flow increase. To accomplish Mach number reduction, the diffuser accepts the impeller discharge flow and directs it through a single or plurality of passages wherein the area and radius increase with the distance along the passage.
Upon discharge from the diffuser, the flow still possesses radial and tangential components of velocity. These components are redirected to an axial direction through a high-radius duct as the flow is delivered to subsequent components. De-swirl vanes may be located in this duct to remove some of the remaining tangential velocity component.
Many configurations are known for steady flow diffusers. For instance, steady flow diffusers are known in the art as vaned diffusers, vane-island diffusers, channel diffusers, cascade diffusers, pipe diffusers, conical diffusers, vaneless diffusers, scroll diffusers, volute diffusers, and the like.
Vaned, vane-island, channel, and cascade diffusers use flat, wedge-shaped, or curved vanes that are arranged to form channels within the diffuser. Each vane provides a pressure surface for one channel and a suction surface for an adjacent channel. The channels are bounded on the two remaining sides by generally parallel solid surfaces that are typically referred to as the hub and shroud surfaces. The two remaining sides of the channel are open so that flow can enter and exit the channel. The pressure and suction surfaces typically diverge to create an increasing flow area along the channel. The increasing area causes a decrease in Mach number as flow moves through the passage. The centerline of each channel is aligned with the absolute angle of impeller discharge flow produced at a particular operating condition. In some situations, the vanes may be rotated so that channel alignment can be maintained at several operating conditions.
The pipe diffuser, or conical diffuser, is a channel-type diffuser where the channel cross section has a circular rather than rectangular shape. The circular cross section in combination with increasing area along the passage gives each passage a conical shape. The leading edge of each passage typically intersects with the leading edge of an adjacent passage creating a scalloped profile.
The flow through a vaneless diffuser is bounded on only the two sides adjacent to the impeller hub and shroud surfaces. The vaneless diffuser is essentially a channel diffuser without vanes. No attempt is made to contain radial or tangential velocity components in a vaneless diffuser. Instead, the flow is allowed to swirl out to higher radii and the tangential velocity is reduced through conservation of angular momentum. Radial velocity is reduced as flow area increases with radius.
The volute or scroll diffuser is formed by a single channel wrapped about the impeller in the direction of rotation. The cross-sectional area of the channel increases with the distance along the flow path. As in the vaneless diffuser, volute diffusers are based on the premise that the angular momentum of the flow remains constant as radius increases. However, flow in the volute diffuser is bounded in all directions except the direction that follows the helical path leading away from the impeller.
The efficiency of any process that uses a centrifugal compressor depends at least partially on the efficiency of the compressor. The efficiency of the compressor depends at least partially on the efficiency of the diffuser in the compressor. A diffuser that loses pressure when converting the kinetic energy of the stream lowers the efficiency of the compressor and thus the efficiency of the process that employs the compressor. Most applications that require centrifugal compressors, especially aircraft gas turbine engine applications, place a size constraint on the compressor. These size limitations cause the outside diameter of the diffuser and discharge duct to be limited. Diffuser performance tends to vary inversely with the level of compactness and thus the size constraints lead to process inefficiencies.
Many systems employing a diffuser require the flow discharged from the diffuser to be substantially parallel to the axis of rotation of the impeller. Components must thus be provided to redirect the flow from the radial and tangential discharge directions to an axial direction. This is commonly achieved with ducts. Such ducts are undesirable because they occupy additional space and typically have a relatively large radius thus increasing the overall diameter of the diffuser. Ducts also lead to pressure losses that lower the efficiency of the diffuser. It is thus desired in the art to provide a centrifugal compressor diffuser that efficiently redirects the impeller discharge flow to an axial flow while maintaining compact overall dimensions. The diffuser should be able to effectively recover kinetic energy from all velocity components present in the impeller discharge flow.
Prior art diffusers employ a steady flow process to accomplish diffusion. The rate at which diffusion and direction changes can take place is limited by natural forces. As flow proceeds through a diffuser passage, the pressure along the passage increases by virtue of the diffusion that occurs along the passage. Concurrently, frictional and viscous forces cause a boundary layer of low energy fluid to develop along the solid surfaces of the diffuser passage. The growth rate of a boundary layer is accelerated by a pressure gradient.
Conditions adverse to effective diffusion result from boundary layer growth. First, boundary layers reduce the flow area available in a passage thus limiting the velocity reduction of the free stream. Second, the kinetic energy of fluid in the growing boundary layer is eventually reduced to the point that it cannot overcome the pressure gradient caused by diffusion. When this occurs, the boundary layer separates from one or more of the solid surfaces and significant pressure losses result. Depending on the structure of the separation, the flow may reverse direction over a portion of the passage and flow back into the impeller. Or, a separation bubble may form that contains recirculating fluid. Both events result in a loss of energy that would otherwise convert to a static pressure increase. The remaining outbound flow is forced through an even smaller flow area that further limits the diffusion that is accomplished. The separated flow eventually leads to a condition that precipitates compressor surge.
Prior design practice recognizes an optimum passage divergence angle of seven to eight degrees. This is a shallow angle that requires a relatively long passage to produce the area ratio needed for effective diffusion. Boundary layer thickness is also a function of passage length. A longer passage increases boundary layer loss. Longer passages also preclude a compact diffuser. A larger divergence angle can be used, but at the risk of causing flow separation. With such constraints in mind, most designers compromise between length and effectiveness. Such a compromise does not recover maximum kinetic energy from the flow.
Diffuser channel alignments and unrecovered kinetic energy result in a tangential component of velocity in the discharge flow. Some designs employ turning vanes in the discharge duct to recover some of this energy. However, boundary layers that develop along the additional tangential distance traveled by the flow, as well as on the turning vanes, cause additional losses.
A radial velocity is also present in a diffuser discharge flow. A duct is required to turn the radial flow to an axial direction for delivery to downstream components. The duct begins at the outermost radius of the diffuser and continues in a radial direction before turning to an axial direction. If the turn is too short, a flow separation will occur along the convex flow surface. More gradual turns require more radial distance working against the size constraint imposed on most diffusers. The gradual turns also produce additional loses as the flow travels through them. It is thus desired in the art to provide a diffuser that redirects the flow without such gradual turns and with reduced boundary layer formation.